Hydroprocessing can include processes which convert hydrocarbons in the presence of hydroprocessing catalyst and hydrogen to more valuable products.
Hydrocracking is a hydroprocessing process in which hydrocarbons crack in the presence of hydrogen and hydrocracking catalyst to lower molecular weight hydrocarbons. Depending on the desired output, a hydrocracking unit may contain one or more beds of the same or different catalyst. Slurry hydrocracking is a slurried catalytic process used to crack residue feeds to gas oils and fuels.
Due to environmental concerns and newly enacted rules and regulations, saleable fuels must meet lower and lower limits on contaminates, such as sulfur and nitrogen. New regulations require essentially complete removal of sulfur from diesel. For example, the ultra low sulfur diesel (ULSD) requirement is typically less than about 10 wppm sulfur.
Hydrotreating is a hydroprocessing process used to remove heteroatoms such as sulfur and nitrogen from hydrocarbon streams to meet fuel specifications and to saturate olefinic compounds. Hydrotreating can be performed at high or low pressures, but is typically operated at lower pressure than hydrocracking. Residue feed from the bottoms of an atmospheric crude column and/or the bottoms of a vacuum crude column may be hydrotreated to make it more suitable for fluid catalytic cracking or other upgrading processes. Diesel and naphtha are produced by hydrotreating residue and must be recovered to recoup value.
Hydroprocessing recovery units typically include a stripper for stripping hydroprocessed effluent with a stripping medium such as steam to remove unwanted hydrogen sulfide. The stripped effluent then is heated in a fired heater to fractionation temperature before entering a product fractionation column to recover products such as naphtha, kerosene and diesel.
Hydroprocessing is very energy-intensive due to the severe process conditions such as the high temperature and pressure used. Over time, although much effort has been spent on improving energy performance for hydroprocessing, the focus has been on reducing reactor heater duty. However, a large heater duty is required to heat stripped effluent before entering the product fractionation column.
There is a continuing need, therefore, for improved methods of recovering fuel products from hydroprocessed effluents. Such methods must be more energy efficient to meet the increasing needs of refiners.